vo_vdpau: add some debugging messages for frame timing

Note that this will print a difference even with perfect sync, because
the code queues the frames _between_ vsync, probably for error margin
(though I don't understand why it uses the exact values chosen).
vo_vdpau: fix unintended truncation of 64 bit timestamps to 32 bit

How embarrassing...

This code is inactive for all VOs other than vo_vdpau. For vo_vdpau,
this caused various issues, such as stuttering after about an hour of
running mpv; see github issue #403.